The big concern about "judicial activism" and the legitimacy of judicial review is that judicial rulings are just politics, unbound by precedents. This concern is particularly true in the U.S. Supreme Court where the Court frequently divides on liberal v. conservative lines in high profile cases. But, the facts are not so simple.
The U.S. Supreme Court is at the extreme pinnacle of the judicial system which is widely acknowledged to have the most political docket of any U.S. court because it is subject to no higher authority whose precedents it must follow on issues of constitutional law. And, it does decide many of the highest profile partisan political legal issues in the land on an almost purely partisan basis. But, these cases make up a surprising modest share of the cases it decides (less than ten percent) and many cases that could conceivably be viewed as having liberal or conservative sides to them are decided on a non-partisan and often on a near consensus basis. What the U.S. Supreme Court does is more like ordinary judging than it is like legislating, even though it sometimes does effectively make law.
There is still a broad baseline of consensus of the substantive meaning of the law (even on issues that divide the more than five dozen lower state and federal appellate courts from which it receives appeals). And, when there is disagreement, the dividing lines after often not partisan political ones.
There is actually a broad near consensus on the Supreme Court among the justices on a supermajority of legal issues that come before the Court. About four out of five cases that come before the U.S. Supreme Court are decided by six more Justices and include justices from both the four liberal and the four conservative Justices on the Court. No two justices disagree more than 30% of the time, and the only disagreement of more than 24% are between Justice Sotomayor with Justices Thomas and Scalia respectively. Chief Justice Roberts disagrees with no other justice more than 20% of the time, and swing Justice Kennedy disagrees with no other Justice more than 21% of the time.
Disagreement among the conservatives on some (by definition, non-partisan) legal issues is almost as high as the partisan disagreement between conservatives and the liberal wing. Conservative justice Alito disagrees with conservative Justice Scalia 23% of the time, and with Justice Thomas 17% of the time, but only disagrees with liberal Justice Breyer 10% of the time. The conservative Justice Thomas disagrees with fellow conservative Chief Justice Roberts 13% of the time, barely less often than the 20% of the time that he disagrees with liberal Justice Breyer. Conservative Justices Thomas and Scalia, however, agree with each other 90% of the time.
The liberal wing of the court is less divided. The three liberal women (Ginsberg, Sotomayor and Kagan) on the Court have very high levels of internal consensus. No justice in this group disagrees with another more than 10% of the time, and Sotomayor and Kagan disagree only 3% of the time. Breyer disagees with his fellow liberal justices 11% to 17% of the time.
SCOTUS Vacancy Prospects
Of course, the partisan cases still matter tremendously and in a body with only nine members who serve for life, every U.S. Supreme Court vacancy matters. For the next four years, President Obama will have the power to nominate the next U.S. Supreme Court justice, although there could be room for politics to delay appointment of a successor until a new President takes office towards the end of his term in late 2016.
As of last June, "Justice Ginsburg, a stalwart of the court’s liberal bloc, has been treated for pancreatic cancer. Justice Antonin Scalia, the court’s most visible conservative, is 76. Justice Anthony M. Kennedy, frequently the swing vote, is 75. And Justice Stephen G. Breyer, like Justice Ginsburg a Democratic appointee, is about to turn 74."
Liberal judges Ginsburg and Breyer, if they are in failing health that they don't think will allow them to survive until the next Democratic President takes office if a Republican wins in the 2016 election, are likely to retire in time to allow President Obama to nominate and have confirmed a liberal successor for their seats. Justices Scalia and Kennedy are less likely to retire voluntarily during President Obama's term of office, but ill health often forces that decision for men of their age. A vacancy in either of their positions during President Obama's term of office would greatly reshape the nation's constitutional and federal jurisprudence, moving it to the left.
Also, a steady stream of Obama appointments to lower courts is already moving federal jurisprudence and the federal courts to subject to the boundaries placed on them by higher courts.